Modeling with reference image
Good Reference Images and Alignment

The biggest problem about modeling 3D models from 2D reference images is the alignment of your reference images - especially when you start modeling faces. 

My advice for a beginner is to start with a "best as possible" aligned reference image. As you can see in the set of images below (from www.3d.sk) I used lines to align the prominent facial features, mainly the eyes, the nose and the mouth (The alignment is still not perfect for various reasons regarding the reference images).



Modeling Process

Although some tutorials on the internet mention that it's best to start with a base mesh for the head, I strongly suggest that you start with a simple plane at the eyes and a mirror modifier, then model the base shape of the face in front view with the least amount of vertices possible. Switch then to side view and start aligning the outer verts to the side reference. 

During the modeling process you are then going to add loop cuts and constantly switch between front and side view to align the vertices.

The best and most in-depth tutorial for beginners on this topic is this one made by Joshua Alger.

Oh, and also do not expect to get a perfectly recognizable 3D representation of the reference images the first time you do this. It'll take some time, but with enough determination you will get there.